The present embodiments relate to a magnetic resonance local coil.
In a magnetic resonance tomography system, conventionally the body to be examined is exposed, with the aid of a basic magnetic field system, to a relatively high basic magnetic field of 3 or 7 tesla, for example. A magnetic field gradient is also applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are then emitted by way of a high-frequency transmission system using suitable antenna devices in order to tip the nuclear spin of atoms that have been excited in a resonant manner by the high frequency field. The nuclear spin of the atoms is tipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field. This high-frequency excitation or the resulting flip angle distribution is also referred to below as nuclear magnetization or “magnetization.” During relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance response signals (“magnetic resonance signals” for short)) are emitted and received by suitable receive antennas and further processed. The raw data acquired in this way may be used to reconstruct the desired image data.
The emission of the high-frequency signals for nuclear spin magnetization may be performed by a “whole-body coil” or “body coil.” A typical design of this coil is a cage-like antenna (e.g., a birdcage antenna) including a plurality of transmit rods arranged running parallel to the longitudinal axis around a patient chamber of the tomography system, in which a patient is present during the examination. At an end face, the antenna rods are respectively connected to each other in a capacitive manner in a ring shape.
Local coils may be used to receive the magnetic resonance response signals from the object under examination. The local coils are receive antenna assemblies including at least one receive antenna element (e.g., in the form of conductor loops). During the examination, the local coils are arranged relatively close to the body surface and if possible, directly on the organ or body part of the patient to be examined. The receive antenna elements may be embodied as a coil. Unlike larger antennas arranged at a greater distance from the patient, local coils have the advantage of being arranged closer to the areas of interest. This reduces the noise component resulting from the electrical losses within the patient's body, which has the result that the signal-noise ratio of the local coil may be better than that of the more remote antenna.
The magnetic resonance signals received by the receive antenna elements may be pre-amplified in the local coil and conducted out of the central region of the magnetic resonance system via cables and sent to a screened receiver in an MRI signal processing device. The received data are digitized and further processed for the imaging.
The cabling of the local coils is, for example, not desired, since the cables cannot be simply run from the patient table to the evaluation device. The cables are perceived as disruptive by the staff, and the patient table with the patient and local coil mat is moved, the cables thus being guided loosely. Therefore, the handling of local coils may be simplified if the data transmission from the local coils to the magnetic resonance tomography system is wireless. It is advantageous, even at the local coil, for the magnetic resonance signals to be provided in analog form and digitized prior to the wireless transmission. Since a circuit of this kind occurs in the “field of view” of the magnetic resonance tomography system (e.g., in the measuring field), the circuit is screened. For example, digital circuits may be susceptible to interfering radiation and may even themselves cause interfering emissions. The function of the circuits may be impaired by the strong field of the high-frequency transmitter for the transmission of the excitation signals. High-frequency emissions emitted by the circuit may be received by the adjacent high-sensitivity receive antenna elements of the local coils and interfere with the reception of the magnetic resonance signals. In addition to improving the electrical properties, the screening may also provide mechanical protection for the circuit.
As standard, digital circuits are screened (e.g., shielded) with an electrically conductive cover connected to a grounding surface. However, in a magnetic resonance device, a particular problem is created by the necessary compatibility between the screening and the alternating fields used. For example, the low-frequency gradient fields that may occur with frequencies of up to 100 kHz may induce unwanted eddy currents in the screening. These eddy currents cause secondary magnetic fields, severe heating due to ohmic losses and vibrations due to Lorentz forces. Shadowing or displacement of the high-frequency fields used during transmission and reception (e.g., the excitation signals and the magnetic resonance signals) may be kept low. The screening may be configured such that these high-frequency fields are not distorted such that the field strength drops in sections.